FIG. 1 depicts a communication device 1 arranged to transmit data in a series of time slots, such as a TDMA system. The communication device 1 comprises a transmitter 2 that includes a power amplifier 3, a power controller 4 arranged to control the power amplifier 3 and a control signal generator 5 that provides a control signal TXC to the power controller 4, together with a TX–RX switch 6 and antenna 7. The power amplifier 3 supplies power to the antenna 7. The antenna 7 is activated at the start of a given time slot and deactivated at the end of that time slot. Of course, TX–RX switch 6 also is connected to a receiver (not shown).
While it is desirable to maximise a portion of the time slot that can be used for data transmission, that is, the portion of the time slot during which maximum power is provided to the antenna 7, the power cannot be raised instantaneously to the maximum level without generating switching transients. In order to limit the generation of switching transients, as required, for example, by the GSM specification, the power supplied to the antenna 7 is gradually ramped up and down at the start and end of each time slot respectively. The time available for deactivating the transmitter at the end of a time slot and reactivating it at the beginning of a following timeslot may be limited. For example, it may be necessary to perform this procedure within a time interval of 33.2 μs, where a time interval of about 12 μs may be required for switching between the two power control loops in the transmitter 2, further reducing the time available for ramping the power amplifier down and up.
Recently, communication devices that are capable of transmitting data using more than one modulation scheme have become available. For example, the Nokia (RTM) 9500 communicator is arranged to send data using EDGE and using GMSK (Gaussian Minimum Shift Keying) modulation on different timeslots. FIG. 2 depicts part of an transmitter 2 suitable for use in such a communication device. FIG. 2 shows a multimode power amplifier 3 and components of the power controller 4.
When signals with a high degree of amplitude modulation are to be transmitted, such as EDGE modulated signals, the power amplifier 3 is operated in a linear mode. The power amplifier 3 is controlled using a first control loop 21, comprising a differential amplifier 8a, a feedback loop including a capacitor 9a and so on. The first control loop 21 controls the output power of the power amplifier 3 by altering an input signal fed to the power amplifier 3, by adjusting the gain of a variable gain amplifier 10.
For modulated signals with little or no amplitude modulation, such as GMSK modulated signals, the power amplifier 3 is operated in a non-linear mode in order to improve its efficiency. When the transmitter is provided in a communication device such as a mobile telephone, this increased efficiency may result in longer talktime. A second control loop 22 is provided, which includes a differential amplifier 8b and a feedback loop including a capacitor 9b. The second control loop 22 controls the power amplifier 3 by altering a voltage applied to a voltage control pin Vpctrl.
The power controller 4 can switch between the two control loops 21, 22 by means of switches 11a, 11b, 11c. 
Ramping of the output power of the amplifier is controlled using a ramp signal TXC generated by the ramp control circuit 5, which follows a regular profile. However, both the control loops 21, 22 include capacitive elements, namely capacitors 9a, 9b, which prevent oscillation. These capacitors 9a, 9b need to be charged to a given level before the output of the power controller 4 can follow the profile of the ramp signal TXC. In FIG. 3, curve A shows an output power against time for an example prior transmitter 2. Curve A begins to follow the ramp signal TXC, not shown, from time t1 onwards, when the output power is 10 dBm or above. This time t1 occurs when the capacitor 9a, 9b in the relevant control loop is sufficiently charged. As shown in the Figure this is preceded by a steep increase in output power, which may give rise to significant switching transients.
The lack of correspondence between the control signal TXC and output power and the resulting increased switching transients arises when the transmitter 2 is initially powered up and when switching between linear and non-linear operational modes, as this involves switching from one control loop to the other.